Added support for optional P4WEB_USER setting.
If set, P4USER will be set
to this user before starting the p4web service. This is recommended, as it
prevents p4web from possibly logging out the primary super user that
operates checkpoints. If defined, the user should also be added to
SDP_AUTOMATION_USERS defined in the Instance Vars file, i.e. the p4_N.vars
file in /p4/common/config. This change is done in a way to avoid disrupting
existing installations. To use the feature, do something like this,
replacing N with the SDP instance name: